Dnatube Scientific Videos

Hair Formation
Hair FormationHair formation begins in the third month of fetal life. Each hair grows in a follicle, which is made from epidermal cells that grow down into the dermis. Hair is created by the multiplication of special cells at the follicle base. A hair develops as the epidermis thickens and cells begin to grow down into the dermis. This invading down-growth forms a cap over some of the connective tissue to create a papilla. Cells of this papilla multiply to form the hair. As these cells are pushed up the central canal of the hair shaft, and thus farther away from their source of nourishment, they become impregnated with the hard protein-- keratin. The cells of the papilla continue to multiply and are successively filled with keratin as the hair grows. Other cells of the papilla form an internal root sheath of keratanized cells. A sebaceous gland develops from cells of the newly formed root sheath. In a fully formed hair follicle, the hair lies within a two-layered hair shaft. The sebaceous gland oils the hair and a small erector muscle is able to make the hair stand on end.
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Added: October 15, 2007, 8:08 pm
Runtime: 72.00 | Views: 6066 | Comments:0
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The Skull
The SkullExcept for the lower jaw, the twenty-nine bones of the skull are fused to form a single unit. Eight of these make up the cranium, enclosing the brain, including the frontal bone, which shapes the forehead and the upper arches of the eye sockets; the two fused parietal bones, which plate the sides and roof of the skull; the occipital bone, which is the back plate of the skull and contains the aperture for the spinal cord; the temporal bones, which provides the lower side walls of the skull and is part of the structure of the ear; the sphenoid bone, a winged structure which runs from eye level to the base of the skull and houses the pituitary gland; and the ethmoid, which is the bone framing part of the eye sockets and the nasal cavity. Fourteen bones shape a framework for the eyes, nose and mouth, they include the Mandible, the largest bone in the face and the only one which is freely movable; the maxillae, which fuse to form the upper jaw; the two lacrimals, the smallest facial bones that make up bone canals for the tear ducts; the nasal bones, two bones that make the bridge of the nose; the zygomatic bones, or the cheekbones which also shape the side walls and the base of the eye sockets; and the teeth. In addition, there are three tiny bones in each ear, and the hyoid bone at the base of the tongue The bones of the skull are tightly joined together by seams of connective tissue called sutures. The skull is particularly vulnerable in early infancy, when there are six membranous gaps, or fontanelles, where the sutures are still incomplete.
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Added: October 15, 2007, 8:10 pm
Runtime: 102.00 | Views: 7273 | Comments:0
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